reserve a,b,c,x,y,z for object,X,Y,Z for set,
  n for Nat,
  i,j for Integer,
  r,r1,r2,r3,s for Real,
  c1,c2 for Complex,
  p for Point of TOP-REAL n;

theorem Th16:
  Sum sqr <*r1,r2,r3*> = r1^2+r2^2+r3^2
  proof
    thus Sum sqr <*r1,r2,r3*> = Sum <*r1^2, r2^2, r3^2*> by Th15
    .= r1^2+r2^2+r3^2 by RVSUM_1:78;
  end;
